An ultrasonic diagnostic apparatus is constructed so as to send and receive ultrasonic waves to and from a body to be examined by means of a probe to obtain information on the body to be examined on the basis of reflected wave signals coming from the body. In this case, in order to obtain images with a high resolving power for any place in different parts at different depths within the body to be examined, dynamic focusing, by which the focal point for received waves is varied dynamically with the lapse of time, is performed at the reception of the reflected waves coming from the body. At this time, the focussing for the received waves described above is effected by adding received signals coming from different vibrating elements to each other by means of a probe, in which a plurality of vibrating elements formed in a narrow strip shape are arranged, or a probe, in which a plurality of annular vibrating elements are arranged coaxially, after having been appropriately delayed by means of a delay line. This circuit is called generally phasing circuit. Since focal points for the received waves are determined by delay times of the delay line, the dynamic focusing can be realized by varying dynamically the delay times, which are to be given to a plurality of received waves, depending on the depth of generation of the reflected waves coming from the body.
The variation of the delay time described above is performed by disposing a plurality of taps with a suitable interval on the delay line, which are selectively switched by means of electronic switches. In this case, it happened that noise was produced at switching the electronic switches described above, which was mixed in the received signals through the delay line, which gave rise to erroneous signals in diagnostics information. In order to remove such a phenomenon, it is sufficient to use switches generating rarely noise for the electronic switches described above. However, since such an electronic switch is expensive, when a number of switches are disposed, one for every tap of the delay line, cost is raised. Therefore this measure has a drawback that it was not economical.
In order to deal with such a problem, a variable delay line, by which the delay time can be controlled continuously by combining inductors with variable capacitance diodes, whose capacitance varies, depending on the magnitude of the reverse bias voltage, has been proposed in JP-B-Sho 60-45830.
The variable delay line disclosed in this publication will be explained referring to FIG. 1. In FIG. 1, a symbol L indicates an inductor; VC a variable capacitance diode; and C a capacitor. A signal input terminal T.sub.1 is disposed at one end and a signal output terminal T.sub.2 is disposed at the other end. In this circuit, capacitance elements in a usual LC delay line are replaced by variable capacitance diodes VC. The anodes of the variable capacitance diodes VC are connected in common, to which an analogue control signal is given through a control terminal T.sub.3. Further then common connecting point is grounded through a capacitor C. In this way the delay time of the variable delay line is controlled by applying an analogue control signal to the control terminal T.sub.3 to vary the reverse bias voltage applied to the variable capacitance diodes VC.
Another variable delay line is described in U.S. Pat. No. 4,481,823, in which inductors L and variable capacitance diodes VC are combined, as indicated in FIG. 2, and on the other hand multi-channel signals are applied to taps disposed with an appropriate interval as a constant current signal source through an amplifier A.sub.0. In FIG. 2, a symbol M indicates a matching circuit for the variable delay line thus constructed.
However, in the variable delay line indicated in FIG. 1 or 2 described above, as it can be seen in the respective figure, when a control signal was applied to the variable capacitance diodes VC to vary the reverse bias voltage for controlling the delay time, it happened that the control signal was mixed in the output signal as a noise component. Consequently there was a problem that the S/N ratio of the outputted ultrasonic signal was decreased and thus an obtained ultrasonic image was warsened.
Further, the variable delay line indicated in FIG. 1 had a problem that signals were remarkably distorted, because the capacitance of the variable capacitance diodes VC was varied by radio frequency signals, when the input signal level was high. With this respect, FIG. 3 shows data measured by using a 10-stage variable delay line provided with e.g. a fixed capacitor whose capacitance is 100 pF and variable capacitance diodes VC whose capacitance varies in a region from 20 pF to 500 pF, when the reverse bias voltage E.sub.c is varied in a region between 1 V and 8 V, in the circuit indicated in FIG. 1. In this variable delay line, as it is clearly seen from the measurement result indicated in FIG. 3, although the distortion factor was as low as about 3% when the level of the input signal was low, the distortion factor reaches a value over 20% when the level of the input signal was high. When such a variable delay line was applied to a phasing circuit in an ultrasonic diagnostic apparatus, it happened that characteristics of the ultrasonic diagnostic apparatus were worsened.
Further, in the case where the delay time is controlled by varying the capacitance of the variable capacitance diodes VC, since the characteristic impedance of this variable delay line is varied simultaneously, a problem that it is mismatched, if matching is effected only by terminating it with a fixed resistance R, takes place too.
Concerning this problem, JP-B-Sho 60-45830 discloses a matching circuit using FETs, as indicated in FIG. 4. Each delay line fi in FIG. 4 has the circuit construction indicated in FIG. 1. It is so constructed that the resistance of the terminating resistor of the delay line fi and the gain of a preamplifier ei or a buffer amplifier gi are varied at the same time as change in the delay time of the respective delay line fi so as to compensate each other. A control section P outputs, apart from a delay time controlling signal for the delay line fi, a control signal for the terminating resistor Z provided with field effect transistors and a gain controlling signals for the Preamplifier ei. These control signals are converted into analog signals by a digital-analogue converter DA and given to the variable capacitance diodes in the delay line fi, the field effect transistors in the terminating resistor Z and the variable gain preamplifier ei, respectively, through a signal distributor MX. In this way, the dynamic focusing is realized by varying the control signals for the delay line fi and the terminating resistance Z in the time so that the focal point obtained by the phasing circuit corresponds to the generating point of the ultrasonic reflected wave. However, since the resistance of the terminating resistor Z described above varied, depending on the ambient temperature, characteristics of the phasing circuit were worsened and therefore the property as an ultrasonic diagnostic apparatus was lowered. Further, if there were fluctuations in characteristics of the field effect transistors used in the terminating resistor Z described above, it was necessary to adjust the device in accordance with the characteristics of the different field effect transistors. Furthermore the resistance of the terminating resistor Z described above varied, depending on the magnitude of the voltage of signals applied thereto, and it happened that the characteristics of the device were worsened, caused by distortions in the signals due to this non-linearity.